False Japanese beetles

Kathleen Bennett, Entomology Technician

Published in Yard & Garden Brief February 2002

Introduction

False Japanese beetles, Strigoderma arbicola, are often confused with Japanese beetles. Also known as spring rose beetles, false Japanese beetles are native to Minnesota. They are found in grassy, sandy areas and are common in Anoka, Washington, Sherburne, Benton, and Stearns counties. Japanese beetles, Popillia japonica, are an introduced species. They are relatively new in Minnesota and are not currently widely distributed in Minnesota. They are potentially serious landscape pests.

Identification and life cycle

False Japanese beetles and Japanese beetles resemble each other. They are both scarab beetles belonging to the same subfamily, shining leaf chafers (Rutelinae). It is important to distinguish between the two insects because the Japanese beetle is a serious landscape pest.

False Japanese beetles overwinter as larvae in the soil at a depth of about 10 inches. The larvae are C-shaped, cream-colored grubs. In the early spring, the beetles pupate and then emerge as adults in late June or early July. The adults are about 7/16 inch long. The false Japanese beetle's head and thorax is a dull, metallic green and its wings are brown. False Japanese beetles mate, lay eggs, and feed in the garden for a couple of weeks and are generally absent from gardens by the end of July.

Japanese beetles also overwinter in the soil as cream-colored larvae and emerge as adults in late June. The adults are about the same size as the false Japanese beetles but are more iridescent with a metallic green head and thorax and shiny coppery brown wings. The main distinguishing feature between the two species is that the Japanese beetle has five tufts of white hair along both sides of the abdomen and two small dots of white hair on the tip of it. Japanese beetles often persist in the garden until the end of August. For more information, see Japanese Beetle Management in Minnesota.

Damage

False Japanese beetle adults typically feed on the blossoms of many plants but may also feed on foliage and fruit. They are especially fond of rose flowers, especially white or light-colored blossoms. These beetles also attack blackberry, clover, coreopsis, hollyhock, honeysuckle, iris, lilies, and peonies and vegetables such as peas, beans, cantaloupe, corn, and cucumbers.

The larvae feed on roots but their plant preferences are not well known. They have been reported to damage roots of corn, potatoes, strawberries, sweet potatoes, and various grasses. However, the larvae are not considered to be pests. In contrast, the Japanese beetles are serious turf pests.

Management

Management methods may depend on population levels of false Japanese beetles and your tolerance for cosmetic damage. Even when management methods are implemented, large populations may overwhelm your garden. False Japanese beetles cause most of their damage within a two-week period.

Physical control
If only a few plants are affected, handpick and kill the adults (e.g., drop them into a pail of soapy water). Since the beetles are strong flyers and can readily move into the garden; continual vigilance throughout July may be necessary.

To prevent flower damage to high value plants, install a light fabric cover such as cheesecloth over the plants.

Chemical control
If physical methods do not provide adequate control, use a residual insecticide such as carbaryl (Sevin) or permethrin (Eight). Even using insecticides may not be completely effective, as insecticide residues may not control any new beetles that fly into the garden. Thus, to conserve beneficial insects, you may want to focus insecticide sprays on high value plants rather than spraying insecticides throughout the yard. The best time to apply an insecticide is during minimal bee activity, which is typically early in the morning or late at night.

CAUTION: Read all label directions very carefully before purchasing and again before using an insecticide. Information on the label should be used as the final authority. Treat only plants listed on the label of the insecticide.